This invention relates to a process for the liquefaction of coal. More particularly, this invention relates to a process whereby coal is depolymerized to a coal extract by catalytic insertion of phenolic materials into the coal structure and to the hydrogenolysis of such a coal extract to produce a stable coal-derived oil.
Liquefaction of coal by depolymerization in an organic solvent has been known for many years. The liquefied coal extract which results from such a process is far more suitable for conversion into liquid fuel by hydrogenolysis than is untreated coal. A wide variety of organic solvents including phenol and related phenolic materials have been advanced as suitable organic solvents for the liquefaction of coal.
U.S. Pat. No. 1,881,927 which issued to Pott et al. in 1932 discloses an early attempt to depolymerize coal with phenol as a solvent and to hydrogenate the resulting liquid coal extract to produce hydrocarbon fuel. In order to achieve sufficient depolymerization with phenol, Pott conducts the depolymerization in several separate extraction steps at temperatures ranging between 320.degree. and 400.degree. C. This procedure requires a treatment time of a few hours. Although Pott discloses a desirable coal extract, the reaction conditions, especially the long holding times, make this and similar depolymerization processes impractical.
More recently, it was discovered that the addition of certain catalysts to the phenol solvent greatly improves the depolymerization of coal solids. The use of catalysts allow a reduction in severity of reaction conditions required to successfully depolymerize coal solids and in some cases permits shortened reaction times. U.S. Pat. No. 3,158,561 to Neuworth et al. discloses one such process in which BF.sub.3 catalyzes depolymerization when it is complexed with the phenolic solvent. Use of the phenolic-BF.sub.3 complex allows the depolymerization reaction to proceed at temperatures of 50.degree. to 200.degree. C. and at autogenous pressure for a period of only about 2 hours. This patent also describes a method of mechanically separating the coal extract from the phenol solvents which involves pouring the coal extract and excess solvents into benzene. When the extract and benzene are combined, all but a small fraction of coal extract precipitates in a granular form which can then be easily separated from the liquid by filtration or centrifugation. The liquid removed during this mechanical separation is vacuum fractionated to separate benzene and phenol from a pitch-like residue which is the benzene soluble fraction of the coal extract.
Despite reported increases in depolymerization efficiency, the BF.sub.3 catalyst has several drawbacks which have prevented wide acceptance by commercial coal processors. BF.sub.3 causes dealkylation of coal with production of alkylphenols, as by products, in fair yield. BF.sub.3 also induces self-condensation of phenol to high molecular weight tars. Both of these side reactions cause undesirable consumption of phenol.
To avoid the difficulties introduced by the BF.sub.3 catalyst, Ouchi et al. have experimented with H.sub.2 SO.sub.4, p-toluene sulfonic acid (CH.sub.3 C.sub.6 H.sub.4 SO.sub.3 H), C.sub.6 H.sub.5 SO.sub.3 H.H.sub.2 O and other Bronsted acids to catalyze the depolymerization of coal by phenol insertion. In an experimental system operating at phenol reflux temperatures and ambient pressures, Ouchi achieves depolymerization in periods from 1 to 50 hours. This work is reported in Fuel 44, 29(1965); 44, 205(1965); 46, 319(1967); 46 397(1967); 52, 156(1973).
The course of reaction whereby BF.sub.3 catalyzes the insertion of phenol into the coal structure does not appear to be the same as Bronsted acid catalysts wherein the hydrogen ion is the actual active catalyst. The catalysts used by Ouchi are a significant improvement over the BF.sub.3 catalyst. The process described by Ouchi, however, is not able to achieve sufficient reaction speed for large continuous flow operations and does not provide for the recovery of costly phenol consumed during the depolymerization reaction.
Darlage and Bailey (Fuel 55, 3(1976)) describe a variety of phenolic solvents which may be used for the depolymerization of coal.